In the related art, spot welding (which is also referred to as “resistance welding”) has been used to join a work obtained by overlapping a plurality of plates. In spot welding, the overlapped work is interposed between a pair of electrode tips and is then pressed by the pair of electrode tips and a current flows between the electrode tips, with the pressure maintained at a predetermined value or more. Then, a work material is melted by Joule heat which is generated by the current and a nugget, which is a piece of molten material of the plates, is generated at the interface between the plates between the electrode tips. Then, the application of the current is stopped while the pressing state is maintained. Then, the nugget is cooled and solidified and the plates are welded.
In the spot welding, in some cases, three or more plates overlap each other and welding is performed on the plates. In this case, the thicknesses of the plates may not be equal to each other. In general, the plates have different thicknesses. For example, in the field of metal plates which are used in vehicles, a work is used which includes a laminate of a plurality of thick and hard metal plates and thin and soft metal plates that are stacked on the outside of the laminate. In the work, the interface between the plate which is the thinnest and is disposed on the outermost side (hereinafter, referred to as the “thinnest plate” in some cases) and the plate adjacent to the thinnest plate is disposed in the vicinity of the outside of the work. Therefore, when welding is performed so that the nugget is generated around the center of the work, the nugget is not sufficiently grown at the interface between the thinnest plate and the adjacent plate and it is difficult to sufficiently join the thinnest plate and the adjacent plate.
In recent years, in order to solve the problems, a spot welding device has been proposed which includes a pair of electrode tips that holds a work between them and an auxiliary electrode tip that comes into contact with the thinnest plate (for example, see Patent Documents 1 and 2). In the spot welding device, the welding electrode tip and the auxiliary electrode tip come into contact with the thinnest plate and a current flows between the electrode tips to heat the vicinity of the outside of the work. Therefore, it is possible to heat the work from the vicinity of the center to the vicinity of the outside and to appropriately weld the work including the thinnest plate.
In resistance welding (which is a synonym of “spot welding”), in some cases, a current is branched at a predetermined ratio and one of the branched currents is cut off by a switch. For example, when the resistance welding is applied to the spot welding device, while a current is flowing between the pair of electrode tips having the work interposed between them and between the welding electrode tip and the auxiliary electrode tip, the current which flows between the welding electrode tip and the auxiliary electrode tip is cut off by the switch to change the position where the welded portion is formed during the resistance welding. A switch may be used as the switch which simply brings two electrodes into contact with each other to carry a current and separates the two electrodes to cut off the current.
However, a large amount of current flows in the resistance welding. Therefore, when the current is cut off, a spark is likely to occur on the surface of the electrode of the switch and the surface of the electrode of the switch is damaged whenever the spark occurs. When the surface of the electrode is damaged, the contact area between the electrodes is changed and the resistance value of the switch is also changed. As a result, a stable current does not flow. A spark prevention mechanism can be provided in order to prevent the occurrence of the spark. However, when a large amount of current flows for resistance welding, it is difficult to completely suppress the occurrence of the spark with the spark prevention mechanism.
In order to prevent the surface of the electrode from being worn, the following electrode has been used (for example, see Patent Document 3): a silver-plated layer is formed on the surface of the electrode; graphite powder is injected to the silver-plated layer at a high speed; the surface of the silver-plated layer is melted by heat generated by the injection; and the graphite powder is stacked onto the surface of the molten silver-plated layer to form a graphite lubrication layer on the electrode.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2011-194464
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2012-11398
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2009-245684